1. Field of the Invention
The present invention relates to a tensioner used for applying appropriate tension to a transmission belt or a transmission chain in a timing system or the like of an engine.
2. Description of the Related Art
It has been common practice to use a tensioner for maintaining appropriate tension in the chain or the like. For example, a known chain guide mechanism, which uses a tensioner lever to slidably guide a transmission chain such as an endless roller chain passing over respective sprockets of a crankshaft and a cam shaft inside an engine room, uses a tensioner for biasing the tensioner lever, in order to maintain appropriate tension.
The known tensioner 510 used in such a chain guide mechanism includes, for example, as shown schematically in FIG. 9, a tensioner body 520 having a cylindrical plunger bore 521 with an open end, a cylindrical plunger 530 in the plunger bore 521 to slide against the cylindrical surface 521a of the plunger bore 521, and biasing unit for biasing the plunger 530 in a direction in which the plunger protrudes. The biasing unit are a coil spring 540 accommodated inside a cylindrical recess 534 in the cylindrical plunger 530 and compressed between the plunger and the bottom 521b of the plunger bore 521.
Oil is supplied from an oil supply hole 525 formed in the tensioner body 520, so that an oil pressure chamber 511 formed between the plunger bore 521 and the plunger 530 is filled with oil. This oil biases the plunger 530 in the protruding direction of the plunger. A check valve 550 (schematically shown only as a check ball) stops the oil from flowing out from the oil supply hole 525. As the plunger 530 reciprocates, oil flows through the small gap between the plunger 530 and the plunger bore 521, and the flow resistance provides the damping effect of slowing down the reciprocal movement of the plunger 530.
In such a known tensioner 510, when let stand for a long time after the oil supply is stopped (in the case with an engine, after the engine is stopped), there is a time lag before oil is supplied immediately after the next start-up, and because the oil in the oil pressure chamber 511 leaks out and no oil is replenished even when the plunger 530 moves back and forth, there occurs a shortage of oil in the oil pressure chamber 511. Because of this shortage of oil, the damping force of the oil does not act on the plunger 530, which sometimes lead to large vibration of and abnormal noise in the transmission chain, or may damage the transmission chain.
Therefore, a tensioner 610 is known (see Japanese Patent Application Laid-open No. 2009-002495, for example), which is designed to have an oil reservoir chamber 631 inside the plunger 630 to supply oil into the oil pressure chamber 611 from the oil reservoir chamber 631, as shown in FIG. 10. A constant amount of oil is retained in the oil reservoir chamber 631 such as not to leak out, so that, even immediately after the start-up after a long stop, oil remaining in the oil reservoir chamber 631 is supplied to the oil pressure chamber 611, to maintain the damping force of oil for the plunger 630 and to prevent vibration or damage of the chain.
In this tensioner 610 described in Japanese Patent Application Laid-open No. 2009-002495, as shown in FIG. 10, an oil supply hole 625 is formed in the tensioner body 620 such as to extend from the outer wall of the tensioner body 620 to the plunger bore 621. A connection/adjustment groove 636 is formed in the outer circumferential surface of the plunger 630 so as to provide an oil passage 635 between the inner circumferential surface of the plunger bore 621 and the outer circumferential surface of the plunger 630. A plunger through hole 632 is formed in the plunger 630 such as to extend from the connection/adjustment groove 636 to the oil reservoir chamber 631 inside the plunger 630. This way, oil is supplied from the outside of the tensioner body 620 into the oil reservoir chamber 631 via the oil supply hole 625, oil passage 635, and plunger through hole 632.